Decoding of Mantle Processes in the Mersin Ophiolite, Turkey, of End-Member Arc Type: Location of the Boninite Magma Generation

The Mersin ophiolite, Turkey, is of typical arc type based on geochemistry of crustal rocks without any signs of mid-ocean ridge (MOR) affinity. We examined its ultramafic rocks to reveal sub-arc mantle processes. Mantle peridotites, poor in clinopyroxene (<1.0 vol.%), show high Fo content of olivine (90–92) and Cr# [=Cr/(Cr + Al) atomic ratio] (=0.62–0.77) of chromian spinel. NiO content of olivine is occasionally high (up to 0.5 wt.%) in the harzburgite. Moho-transition zone (MTZ) dunite is also highly depleted, i.e., spinel is high Cr# (0.78–0.89), clinopyroxene is poor in HREE, and olivine is high Fo (up to 92), but relatively low in NiO (0.1–0.4 wt.%). The harzburgite is residue after high-degree mantle melting, possibly assisted by slab-derived fluid. The high-Ni character of olivine suggests secondary metasomatic formation of olivine-replacing orthopyroxene although replacement textures are unclear. The MTZ dunite is of replacive origin, resulted from interaction between Mg-rich melt released from harzburgite diapir and another harzburgite at the diapir roof. The MTZ dunite is the very place that produced the boninitic and replacive dunite. The MTZ is thicker (>1 km) in Mersin than in MOR-related ophiolite (mostly < 500 m), and this is one of the features of arc-type ophiolite.


Introduction
The mantle wedge is the very place where both magmatism and metasomatism are active, and is considered to be complicated in petrologic nature (e.g., [1]). It is quite important to accumulate petrologic data for our better understanding of the mantle wedge. Mantle peridotite xenoliths from current island arcs have been a good source of information about the mantle wedge because of the absence of serpentinization, but their small sizes (<1 m across) and fragmentary nature lead to a serious disadvantage in terms of spatial resolution of mantle processes. The mantle section of ophiolites of arc origin may provide us with information to delineate details of the mantle wedge processes because of its greater exposure. For example, the Oman ophiolite has a mantle section of~500 km in length [2]. However, ophiolites have various origins (e.g., [3]), and therefore, we should carefully select ophiolites for the purpose of investigation of the mantle wedge process. Typical ophiolites including the Tethyan  [8]. (b) Simplified geological map of the Mersin ophiolite [8]. R1 and R2 are routes for taking samples for this study and the details are in Figure 2.
The Mersin ophiolite is situated in the Southern flank of the Tauride platform and located approximately 30 km NW of Mersin town in Southern Turkey (Figure 1a). It is exposed in an area of 60 km long-25 km wide and displays an approximately 6 km-thick crust-mantle section (Figure 1b). The Mersin ophiolite shows a good ophiolitic sequence except for the absence of sheeted dykes, and the stratigraphy is as follows from bottom to top: Mantle tectonites with an underlying metamorphic  [8]. (b) Simplified geological map of the Mersin ophiolite [8]. R1 and R2 are routes for taking samples for this study and the details are in Figure 2.  Table 1.

Sampling
In the Mersin ophiolite, the upper part of the ophiolitic sequence (effusive rocks, gabbros, ultramafic, and mafic cumulates) is available in the Westernmost area (Figure 1b), while the lower part (mantle tectonite) with the underlying metamorphic sole, is exposed mainly in the North and

Sampling
In the Mersin ophiolite, the upper part of the ophiolitic sequence (effusive rocks, gabbros, ultramafic, and mafic cumulates) is available in the Westernmost area (Figure 1b), while the lower part (mantle tectonite) with the underlying metamorphic sole, is exposed mainly in the North and Eastern regions (Figure 1b Table 1). We call ultramafic lithologies from the upper section downward as follows: Layered ultramafic (LU) rocks for the cumulates of Parlak et al. [9], Moho-transition zone (MTZ) peridotites for the mantle tectonite of Parlak et al. [9] along the Sorgun valley (R1), and mantle peridotites for the mantle tectonite of Parlak et al. [9] along the Karakuz valley, respectively (cf. Figure 1b). Along the Sorgun valley, R1, we observed good exposure of MTZ peridotite with minor amount of ultramafic and mafic-layered rocks (LU rocks) (Figures 1b and 2; [30]) and the total thickness of the layered rocks is over 3 km [30]. The Mersin mélange is underneath the ophiolite, and there is no metamorphic sole in this region (Figures 1b and 2; [30]). The detailed petrological and geochemical descriptions of the mafic-ultramafic-layered rocks are in [29,30]. Along the Karakuz valley, R2, we observed mantle peridotites (mainly harzburgite and dunite), although it is not clear stratigraphically how lower these mantle peridotites are than the base of the MTZ peridotites of R1 [29,30] (Figure 1b).
We started sampling from the clinopyroxenite of the LU rocks (upper) to harzburgite of the MTZ peridotites (lower) in R1 (Figures 1b, 2 and 3; Table 1). Clinopyroxenite and wehrlite of the LU rocks show nice layering (Figure 4a). Most of the MTZ peridotites are dunite with a minor amount of harzburgite and wehrlite. Harzburgite occurs only just above the Mersin mélange and coexists with dunite in R1. However, we could not observe their relationships on the outcrop because of high degrees of shearing and serpentinization (Figures 2a and 3). Chromite layers were observed in both the LU rocks and the MTZ dunite (Figure 4b), and their strikes and dips are about N82 • E84 • S in the LU rocks and N80 • E78 • S in the MTZ rocks ( Figure 2a; Table 1). There is a thick dolerite dike in dunite of MTZ peridotites, and the strike is discordant to the chromitite layer.  Table 1). We call ultramafic lithologies from the upper section downward as follows: Layered ultramafic (LU) rocks for the cumulates of Parlak et al. [9], Moho-transition zone (MTZ) peridotites for the mantle tectonite of Parlak et al. [9] along the Sorgun valley (R1), and mantle peridotites for the mantle tectonite of Parlak et al. [9] along the Karakuz valley, respectively (cf. Figure  1b). Along the Sorgun valley, R1, we observed good exposure of MTZ peridotite with minor amount of ultramafic and mafic-layered rocks (LU rocks) (Figures 1b and 2; [30]) and the total thickness of the layered rocks is over 3 km [30]. The Mersin mélange is underneath the ophiolite, and there is no metamorphic sole in this region (Figures 1b and 2; [30]). The detailed petrological and geochemical descriptions of the mafic-ultramafic-layered rocks are in [29,30]. Along the Karakuz valley, R2, we observed mantle peridotites (mainly harzburgite and dunite), although it is not clear stratigraphically how lower these mantle peridotites are than the base of the MTZ peridotites of R1 [29,30] (Figure 1b). We started sampling from the clinopyroxenite of the LU rocks (upper) to harzburgite of the MTZ peridotites (lower) in R1 (Figures 1b, 2 and 3; Table 1). Clinopyroxenite and wehrlite of the LU rocks show nice layering (Figure 4a). Most of the MTZ peridotites are dunite with a minor amount of harzburgite and wehrlite. Harzburgite occurs only just above the Mersin mélange and coexists with dunite in R1. However, we could not observe their relationships on the outcrop because of high degrees of shearing and serpentinization (Figures 2a and 3). Chromite layers were observed in both the LU rocks and the MTZ dunite (Figure 4b), and their strikes and dips are about N82°E84°S in the LU rocks and N80°E78°S in the MTZ rocks ( Figure 2a; Table 1). There is a thick dolerite dike in dunite of MTZ peridotites, and the strike is discordant to the chromitite layer.   Table 1. Abbreviations are the same as Figure 2.  In R2, we collected harzburgite and dunite of mantle peridotite with minor amount of pyroxenites, i.e., clinopyroxenite, orthopyroxenite and orthopyroxene-rich olivine websterite (Figures 2 and 3). The pyroxenites are mainly observed as discordant dike, cross-cutting the harzburgite foliation. The Northern end of the R2 is interpreted as a stratigraphically lower section of mantle peridotites based on field observation of strikes and dips of harzburgite foliation (Figures  2b and 3b; Table 1). Some dunites have coarse clinopyroxene patches and/or seams (Figure 4c), and are discordant to the harzburgite foliation (Figure 4d,e). Orthopyroxene gradually decreases in mode from harzburgite to dunite at their boundary. Banded chromitite was observed also in the R2 mantle dunite. In R2, we collected harzburgite and dunite of mantle peridotite with minor amount of pyroxenites, i.e., clinopyroxenite, orthopyroxenite and orthopyroxene-rich olivine websterite (Figures 2 and 3). The pyroxenites are mainly observed as discordant dike, cross-cutting the harzburgite foliation. The Northern end of the R2 is interpreted as a stratigraphically lower section of mantle peridotites based on field observation of strikes and dips of harzburgite foliation (Figures 2b and 3b; Table 1). Some dunites have coarse clinopyroxene patches and/or seams (Figure 4c), and are discordant to the harzburgite foliation (Figure 4d,e). Orthopyroxene gradually decreases in mode from harzburgite to dunite at their boundary. Banded chromitite was observed also in the R2 mantle dunite.

Petrographic Features of Ultramafic Rocks
The samples are highly serpentinized (up to 96 vol.%) and the degree of serpentinization is lower in wehrlite (60~80 vol.%) and harzburgite (50~87 vol.%) than in dunite (71~96 vol.%). We determined their modal compositions using point counter (~2000 counts in total) and the primary lithology was determined all serpentine is the secondary phase after olivine except for pseudomorphs of pyroxenes ( Figure 5). If there is pyroxene pseudomorph with orthogonal and oblique extinction, we accounted for the pseudomorph as orthopyroxene and clinopyroxene, respectively.

Petrographic Features of Ultramafic Rocks
The samples are highly serpentinized (up to 96 vol.%) and the degree of serpentinization is lower in wehrlite (60~80 vol.%) and harzburgite (50~87 vol.%) than in dunite (71~96 vol.%). We determined their modal compositions using point counter (~2000 counts in total) and the primary lithology was determined all serpentine is the secondary phase after olivine except for pseudomorphs of pyroxenes ( Figure 5). If there is pyroxene pseudomorph with orthogonal and oblique extinction, we accounted for the pseudomorph as orthopyroxene and clinopyroxene, respectively. We assumed all serpentine as secondary phase after olivine unless obvious feature of pseudomorphs after pyroxenes, e.g., cleavage, was recognized (see text). Dashed line represents clinopyroxene (cpx)/orthopytoxene (opx) = 1:9 ratio, indicating the possible presence of clinopyroxene at solidus temperatures above this line [35].

MTZ Peridotites (Dunite-Wehrlite-Harzburgite of R1)
All types of lithology in this group show higher degree of serpentinization than equivalents of the LU rocks (Figure 6a,b).
Mineral assemblage of MTZ dunite is the same as that of the LU rocks and the degree of serpentinization is higher (89.6~95.5 vol.%) than in the LU rocks. The relationship between olivine and clinopyroxene is, however, unclear due to high degree of serpentinization. Modal amount of olivine is quite high (>97 vol.%), and chromian spinel is higher in mode (0.9~2.5 vol.%) than in the LU rocks. Some olivine grains show wavy extinction and kink banding (Figure 6b). Modal amount of clinopyroxene (<1.0 vol.%) is lower than in the LU rocks, and the size is also small (up to 500 µm). Chromian spinel is euhedral to subhedral and rarely contains inclusions.
Massive chromitite (14T-MR01) is mainly composed of chromian spinel with minor amount of chlorite and serpentine. Olivine is completely serpentinized or altered. Chlorite occurs solely around chromian spinel or inside as inclusions. Amphiboles also occasionally occur as inclusion in chromian spinel.
We found wehrlite at a few outcrops in MTZ (Table 1), and its mineral assemblage is the same as that in the LU rocks. Modal amount is varied (9.9~14.2 vol.%) for clinopyroxene and almost the constant (0.4~0.9 vol.%) for chromian spinel. Grain size of clinopyroxene is almost the same as that in the LU rocks and the shape is subhedral. Chromian spinel grains show euhedral to subhedral shapes.
MTZ harzburgite is also highly serpentinized (79.6 vol.%), and calculated modal amounts of olivine, orthopyroxene, chromian spinel are 89.7 vol.%, 10.1 vol.%, and 0.2 vol.%, respectively. Clinopyroxene is fine in size (<100 µm) and very small in amount. Orthopyroxene, partly to completely altered, show two types: One shows rounded shapes and the other showing subhedral shape, of which rim is interstitial to olivine grains. The orthopyroxene is usually coarse (up to 1 mm) and there is no obvious relationship between the shape and the size. Chromian spinel is opaque and shows subhedral to anhedral shapes in thin section.

Mantle Peridotites (Ultramafic Rocks of R2)
Mantle dunite in R2 is less serpentinized (71.4~88.7 vol.%) than that in R1, but the modal amounts of clinopyroxene and chromian spinel are not so different between the two (0.3~0.9 vol.% and 1.3~2.4 vol.%, respectively). The size of clinopyroxene is far smaller (≈100 µm) than in dunites from the LU rocks and MTZ, but rarely large (≈1 mm). Olivine is usually coarse (up to 5 mm), and shows weak wavy extinction. Chromian spinel grains show euhedral to subhedral rounded shapes, and rarely contains inclusions of hornblende + phlogopite ± clinopyroxene (Figure 6c).
Orthopyroxene-rich olivine websterite and olivine orthopyroxenite is composed of orthopyroxene, clinopyroxene and olivine with a minor amount of chromian spinel. Orthopyroxene is very coarse (up to 2 cm) and highly kinked but partly altered. Olivine and clinopyroxene are basically small (~500 µm). Amphiboles occur as lamellar inclusions after clinopyroxene lamellae in orthopyroxene, as well as discrete grains forming veinlets. Chromian spinel is subhedral and rounded in shape.
The mantle harzburgite is the same in mineral assemblage as that of MTZ harzburgite but is lower in the degree of serpentinization (50.6~79.2 vol.%) than the MTZ harzburgite. There is no obvious relationship between the degree of serpentinization or modal amounts of pyroxenes and the stratigraphical position (=depth from the MTZ); modal amounts of orthopyroxene and clinopyroxene vary from 12.6 to 21.5 vol.% and from a trace to 0.7 vol.%, respectively ( Figure 5). Some of olivine and orthopyroxene grains show wavy extinction and kink banding (Figure 6d). The orthopyroxene is the same in shape as that in MTZ harzburgite, and it is noteworthy that some orthopyroxene grains appears to consuming coarse olivine grains (Figure 6d). Chromian spinel shows subhedral to anhedral shapes.
Banded chromitites show graded bedding, and olivine and clinopyroxene are in part preserved in a spinel-poor zone. In this case, chlorite is absent around chromian spinel, although present as inclusions in spinel. Olivine and clinopyroxene also occur as inclusion in chromian spinel, and clinopyroxene rarely coexists with phlogopite, chlorite, apatite and sulfide in individual inclusions. Fe-Ni alloys are present in serpentinized matrix in a spinel-rich zone.

Analytical Methods
We determined major-and trace-element compositions of minerals by using an electron probe micro analyzer (EPMA; EPMA-1720H, SHIMADZU) of Engineering Research Equipment Center at Kumamoto University, and a laser (193 nm ArF excimer; GeoLas Q-plus, MicroLas) ablation-inductively coupled plasma-mass spectrometer (Agilent 7500s, Agilent Technologies, Santa Clara, CA, USA) (LA-ICP-MS) at Kanazawa University, respectively. For the EPMA analyses, the accelerating voltage, prove current, and probe diameter were 15 kV, 20 nA, and 5 µm, respectively. We used mineral and oxide standards (SiO 2 , KTiPO 4 , NaAlSi 2 O 6 , Cr 2 O 3 , Fe 2 SiO 4 , MnO, MgO, CaSiO 3 , and NiO) for calibration, and the counting time for each peak and background position was 10 s for both. For LA-ICP-MS analyses, we used 50 µm diameter spot for clinopyroxene and 100 µm diameter spot for orthopyroxene at a 6 Hz repetition rate with an energy density of 8 J/cm 2 per pulse. The NIST SRM 612 was used for external calibration standard and 29 Si as an internal standard, based on the SiO 2 contents obtained from EPMA analyses. The details of analytical methods, including data precision and accuracy, were in Morishita et al. [36].

Major-Element Compositions
The results are shown in Figures 7-10 and Tables 2-5. We mainly used three parameters in following sections, Mg# for all minerals, and Cr# and Y Fe for chromian spinel, and are Mg/(Mg + total Fe) atomic ratio, Cr/(Cr + Al) atomic ratio, and Fe 3+ /(Al + Cr + Fe 3+ ) atomic ratio, respectively. For calculation of Mg# for chromian spinel, we calculated ferric iron content based on spinel stoichiometry and Mg# for chromian spinel is Mg/(Mg + Fe 2+ ) atomic ratio. Orthopyroxene and clinopyroxene frequently occur as coarse porphyroclasts (>1 cm), and show no obvious chemical zoning. We also determined compositions of some amphiboles and used definition by Leake et al. [37] for their classification.  Table 5). Tremolites occur as inclusion in chromian spinel in dunite or as discrete grains in clinopyroxenite (2013MR2-8 and 2013MR2-10). Table 2. Averaged values of major element compositions of olivine in each sample from the Mersin ophiolite.     Table 3. Averaged values of major element compositions of chromian spinel in each sample from the Mersin ophiolite.       If there are different lithological parts within one sample, it is noted, e.g., (Harz part). Abbreviations of rock lithology are the same as Figure 2. FeO* is total iron oxide content calculated by assuming all iron is Fe 2+ . Mg#, Mg/(Mg + Fe total) atomic ratio. Blank means below their detection limits.        [38] and mantle peridotite olivine (blue) [39] show the compositional variation of residual mantle peridotite. (b) Chemical variations between Fo content of olivine and Cr# of chromian spinel. Abyssal peridotite is shown for comparison [40]. OSMA shows the residual trend of spinel peridotite [40]. Tie-lined plots of mantle harzburgite and dunite is the sample from the boundary (2014MR2-09) (Tables 1-3). Arrows are showing evolution trends of the Mersin ultramafic rocks, of melting (blue), reaction (red), and fractionation (black), respectively, which are suggested by mineral chemistry (see discussion).  [43].

Trace-Element Compositions
We determined trace-element compositions of clinopyroxene for each sample and orthopyroxene, if any, i.e., harzburgite and pyroxenites, and the results are shown in Tables 6 and 7 and Figures 11 and 12. Minerals 2017, 7, x FOR PEER REVIEW 21 of 33

Trace-Element Compositions
We determined trace-element compositions of clinopyroxene for each sample and orthopyroxene, if any, i.e., harzburgite and pyroxenites, and the results are shown in Tables 6 and 7 and Figures 11 and 12. Figure 11. REE patterns, which are normalized to chondrite values [44], of clinopyroxene and the calculated melt in equilibrium with clinopyroxene (calc. melt). Abyssal peridotites [45][46][47] in panel (d,f) are for reference. Data vacancy means abundances below the detection limits (see Table 6). (a) LU dunite (black), wehrlite (orange) and clinopyroxenite (green). Calculated melt in equilibrium with clinopyroxene (gray field) in all rocks are almost the same in pattern and shown in gray color all together. (b) Calculated melt in equilibrium with clinopyroxene in dunite, wehrlite, clinopyroxenite, and websterite/orthopyroxenite is shown for each section. Patterns of the Mersin tholeiitic basalt (black) [9] and boninite (BON) from Izu-Ogasawara arc (violet) [48] are for reference. (c) MTZ dunite (black) and wehrlite (orange). Calculated melts in equilibrium with clinopyroxene in wehrlite and dunite are black field outlined by orange and gray field outlined by red, respectively. (d) MTZ harzburgite. (e) Mantle dunite (black) and websterite/orthopyroxenite (dark orange). Calculated melt in equilibrium with clinopyroxene in websterite/orthopyroxenite is within the range of those in dunite and not specifically shown (field filled by oblique stripe). (f) Mantle harzburgite. Figure 11. REE patterns, which are normalized to chondrite values [44], of clinopyroxene and the calculated melt in equilibrium with clinopyroxene (calc. melt). Abyssal peridotites [45][46][47] in panel (d,f) are for reference. Data vacancy means abundances below the detection limits (see Table 6). (a) LU dunite (black), wehrlite (orange) and clinopyroxenite (green). Calculated melt in equilibrium with clinopyroxene (gray field) in all rocks are almost the same in pattern and shown in gray color all together. (b) Calculated melt in equilibrium with clinopyroxene in dunite, wehrlite, clinopyroxenite, and websterite/orthopyroxenite is shown for each section. Patterns of the Mersin tholeiitic basalt (black) [9] and boninite (BON) from Izu-Ogasawara arc (violet) [48] Figure  11a). There is a weak correlation between clinopyroxene REECN abundances and the stratigraphical position of host rock; the REECN of clinopyroxene roughly decreases down-section (Table 6). Other incompatible elements are mostly below their detection limits in abundance, except for Nb, Pb, Sr, and Zr, and there is no systematic increase or decrease upsection or downsection (Table 6).
Chondrite normalized REE patterns of orthopyroxene in clinopyroxenite of the LU rocks show slightly steeper slope from Lu to Dy than those of coexisting clinopyroxene and most of LREE~middle  (Figure 11a). There is a weak correlation between clinopyroxene REE CN abundances and the stratigraphical position of host rock; the REE CN of clinopyroxene roughly decreases down-section (Table 6). Other incompatible elements are mostly below their detection limits in abundance, except for Nb, Pb, Sr, and Zr, and there is no systematic increase or decrease upsection or downsection (Table 6).
Chondrite normalized REE patterns of orthopyroxene in clinopyroxenite of the LU rocks show slightly steeper slope from Lu to Dy than those of coexisting clinopyroxene and most of LREE~middle REE (MREE), except for La and Ce, are below their detection limits in abundance ( Figure 12). Most of other incompatible trace elements were not detected, and if detected, the abundances are almost the same as or lower than in coexisting clinopyroxene (Table 7).

MTZ Peridotites (Dunite-Wehrlite-Harzburgite of R1)
Abundances of some REE in clinopyroxenes in MTZ peridotites are below their detection limits and almost the same as those in the LU rocks (Figure 11a,c). REE patterns of clinopyroxene in MTZ wehrlite are almost constant and show the same shape and abundances as in LU wehrlite (Figure 11a,c). Clinopyroxenes in some MTZ dunite samples show slightly different REE CN patterns with a hump at MREE to heavy REE (HREE) and a peak at Ho to Er (Figure 11c). REE patterns of clinopyroxenes show slightly steeper slopes from HREE to MREE in MTZ harzburgite than in MTZ dunite (Figure 11c,d), although the abundances are almost the same. Other incompatible elements are mostly below their detection limits except for Ba, Nb, Pb, Sr, and Zr as those in the LU rocks (Table 6).
REE CN patterns of orthopyroxene in MTZ harzburgite show steeper slope from Lu to Dy and lower abundances than those of coexisting clinopyroxene (Figure 12). Most of other incompatible trace elements are below their detection limits, and if detected, the abundances are almost the same as or lower than coexisting clinopyroxene as in clinopyroxenite of the LU rocks ( Table 7). The concentrations of some large ion lithophile elements (LILE: Rb, Ba, and Sr) in these orthopyroxenes are slightly higher than those of mantle harzburgite, as shown in the following Section 4.3.3 (Table 7).

Mantle Peridotites (Ultramafic Rocks of R2)
REE CN patterns of clinopyroxenes in mantle dunite are highly varied and roughly divided into two types: One shows simple gentle slope from HREE to LREE and the other, LREE-enriched spoon shaped pattern (Figure 11e). The shape of the former, as well as abundances, is almost the same as those in LU rocks and MTZ wehrlite (Figure 11a,c). On the other hand, the latter shows higher REE CN abundances and some show some flattened patterns (Figure 11e). The REE CN patterns of clinopyroxene in mantle websterite/orthopyroxenite are within the variation of those in dunite (Figure 11e). REE CN pattern of clinopyroxenes in mantle harzburgite is almost the same, although the LREE abundance is slightly higher, as that in MTZ harzburgite (Figure 11d,f). The REE CN abundance of these clinopyroxenes is obviously low relative to those of abyssal peridotites (Figure 11e). There is no clear correlation between the REE abundance of clinopyroxenes and the stratigraphical position of host rock (Table 6). Other incompatible elements are mostly below their detection limits except Nb, Pb, Sr, and Zr as those in the rocks of MTZ harzburgite (Table 6).
REE CN patterns of orthopyroxene in mantle harzburgite show almost the same as those in MTZ harzburgite, although the abundances are slightly lower in the former ( Figure 12). Most of other incompatible trace elements are below their detection limits, and if detected, the abundances are almost the same or lower than coexisting clinopyroxene as in other samples (Table 7).      Table 6. Cont.  Italic values in parentheses are shown their detection limits when analyzed data below their detection limits. If there are different lithological parts within one sample, it is noted, e.g., (Harz part). Abbreviations of rock lithology are the same as Figure 2. Table 7. Representative trace-element compositions of orthopyroxene in the Mersin ultramafic rocks.

Partial Melting Process Recorded in Mantle Harzburgite
Harzburgite samples from both MTZ and mantle peridotite zones show depleted features, high Fo content of olivine (90-91) and high Cr# of spinel (0.62-0.77), and are plotted in the trend of OSMA (Figure 7b). The Cr# of chromian spinel is an indicator of degree of partial melting for residual spinel peridotite or primary melt composition for cumulative ultramafics and volcanic rocks (e.g., [49][50][51]). It is known to be resistant against alteration and weathering, although changeable via subsolidus formation of Al-rich phase or metasomatism (e.g., [50,52]). In the Mersin's case, there is no obvious formation of subsolidus Al-rich phase around the spinel, e.g., plagioclase (and its alteration product), in both MTZ and mantle harzburgites. There are abundant discordant dunite and pyroxene-rich dikes in the mantle section (cf. Figure 2 Table 3). These lines of evidence indicate that the chromian spinel in the Mersin harzburgites has kept its initial chemical features, apart from subsolidus Mg-Fe 2+ exchange, after their partial melting. The Mersin harzburgites are the residual peridotite after a substantially high degree of melting compared to the general abyssal peridotites (e.g., [50]). However, the Fo content of olivine of the Mersin harzburgite shows slightly lower values and some harzburgites are off the partial melting trend of spinel peridotite [40] (Figure 7b). The Fo content of olivine, the Cr# of spinel and the HREE contents of clinopyroxene in harzburgite normally show simple depletion trend with increasing the degree of melting (e.g., [50,53]). The Cr# of spinel shows a simple increase with a decrease of HREE in clinopyroxenes in harzburgites (Figure 13d). Note that the LREE content of clinopyroxene shows no clear correlation with other parameters (Figure 13a,b). These features indicate that partial melting had occurred possibly under the presence of LREE-rich influx. No negative correlation between HREE content of clinopyroxene and Fo content of olivine (Figure 13c) implies the addition of Fe during partial melting. In addition, high concentration of some LILE (e.g., Sr and Pb) are possibly resulted from the addition of such elements during the partial melting (Table 6). Aqueous fluid, which is rich in LREE, LILE, and Fe, is a candidate as the influx, and high Cr# of spinel in the Mersin harzburgite (up to 0.8) is consistent with the high-degree partial melting under hydrous condition (e.g., [54]). Then, the harzburgite of the Mersin ophiolite is possibly residue after high-degree partial melting assisted by influx of aqueous fluid(s) or hydrous melt(s), which were rich in Fe, LILE, and LREE.
To estimate the degree of partial melting of residual mantle peridotite, HREE concentrations in clinopyroxene are quite useful (e.g., [53,55]) because they are relatively unchangeable during metasomatism and weathering, although they could be slightly lowered by formation of subsolidus clinopyroxenes and subsolidus trace-element redistribution with other peridotitic minerals (e.g., [56,57]). Here we do not estimate the subsolidus effects for REE in clinopyroxene and orthopyroxene, because such subsolidus redistribution effect for HREE is relatively small. We roughly estimated the degree of melting by using HREE compositions of clinopyroxene for both the MTZ and mantle harzburgites, to be approximately 25% melting of depleted MORB mantle (=DMM; [58]) at spinel stability field. We also checked the possibility of melting at garnet stability field. The calculated melting degree of DMM at the garnet stability field is mostly 5% followed by 18-20% melting of the residue at the spinel stability field. The HREE concentrations of orthopyroxene are also a good measure for the partial melting event (e.g., [59]). Calculated degree of melting of DMM by using HREE composition of orthopyroxene is 10% at garnet stability field and 10% at spinel stability field. These values are consistent with the high Cr# (up to 0.8) of chromian spinel in the Mersin harzburgite, which could be attained by a relatively high degree (>~20%) of melting of peridotite with an influx of H 2 O-bearing fluid or melt (e.g., [54,60]).

Metasomatism by SiO2-Rich Fluid/Melt Suggested by High-Ni Olivine of Harzburgite
Some olivines in harzburgites both from MTZ and from the mantle section show obviously high NiO content off the "mantle olivine array" (Figure 7a). To increase NiO content of olivine, Ni addition is needed and could be supplied from surrounding olivine, replaced by secondary orthopyroxene (cf. Figure 6d) due to infiltration of SiO2-rich melt or fluid: 2(Mg, Fe, Ni)2SiO4 (olivine) + SiO2 (melt/fluid) = 2(Mg, Fe)SiO3 (orthopyroxene) + Ni2SiO4 (Ni-rich olivine component) [61]. This is due to a > two times higher distribution coefficients of Ni between silicate melt and olivine than those between silicate melt and orthopyroxene [62]. However, local Ni enrichment has not been observed even on olivine grains adjacent to any orthopyroxene grains in individual samples. These imply that such a metasomatic event was relatively old and the local metasomatic Ni enrichment in olivine obscured by subsequent partial melting of the metasomatized peridotite ( Figure 14). The subsequent partial melting of the metasomatized peridotite consumed both orthopyroxenes, i.e., primary and secondary, and left no obvious texture of orthopyroxene enrichment in the Mersin harzburgite (cf. Figure 5), which is in contrast to the clear replacement textures of secondary orthopyroxene formed by young metasomatism in sub-arc peridotite xenoliths (e.g., [1]).

Nature and Formation Process of Dunites, Wehrlite and Pyroxenites
As for the origin of dunite, we have three possibilities: i.e., (1) Residual dunite after extremely high degree of partial melting of peridotite (residual origin); (2) a cumulus dunite crystallized from primitive mantle melt; and (3) a replacive dunite formed by interaction between residual mantle harzburgite and upwelling mantle melt. If the dunite is of residual origin, it is expected to occur at the uppermost mantle and to show extremely depleted mineral chemistry, i.e., high Cr# of spinel, high Fo content of olivine (e.g., [63]) and low incompatible trace element concentrations of clinopyroxene (e.g., [64]). Clinopyroxenes in LU dunites show slightly higher REE abundances than in the both harzburgites of MTZ and mantle section ( Figure 11). In addition, the LU dunite shows adcumulate and mesocumulate textures [29] and the mineral chemistry, e.g., abrupt decrease of NiO

Metasomatism by SiO 2 -Rich Fluid/Melt Suggested by High-Ni Olivine of Harzburgite
Some olivines in harzburgites both from MTZ and from the mantle section show obviously high NiO content off the "mantle olivine array" (Figure 7a). To increase NiO content of olivine, Ni addition is needed and could be supplied from surrounding olivine, replaced by secondary orthopyroxene (cf. Figure 6d) due to infiltration of SiO 2 -rich melt or fluid: 2(Mg, Fe, Ni) 2 SiO 4 (olivine) + SiO 2 (melt/fluid) = 2(Mg, Fe)SiO 3 (orthopyroxene) + Ni 2 SiO 4 (Ni-rich olivine component) [61]. This is due to a > two times higher distribution coefficients of Ni between silicate melt and olivine than those between silicate melt and orthopyroxene [62]. However, local Ni enrichment has not been observed even on olivine grains adjacent to any orthopyroxene grains in individual samples. These imply that such a metasomatic event was relatively old and the local metasomatic Ni enrichment in olivine obscured by subsequent partial melting of the metasomatized peridotite ( Figure 14). The subsequent partial melting of the metasomatized peridotite consumed both orthopyroxenes, i.e., primary and secondary, and left no obvious texture of orthopyroxene enrichment in the Mersin harzburgite (cf. Figure 5), which is in contrast to the clear replacement textures of secondary orthopyroxene formed by young metasomatism in sub-arc peridotite xenoliths (e.g., [1]).

Nature and Formation Process of Dunites, Wehrlite and Pyroxenites
As for the origin of dunite, we have three possibilities: i.e., (1) Residual dunite after extremely high degree of partial melting of peridotite (residual origin); (2) a cumulus dunite crystallized from primitive mantle melt; and (3) a replacive dunite formed by interaction between residual mantle harzburgite and upwelling mantle melt. If the dunite is of residual origin, it is expected to occur at the uppermost mantle and to show extremely depleted mineral chemistry, i.e., high Cr# of spinel, high Fo content of olivine (e.g., [63]) and low incompatible trace element concentrations of clinopyroxene (e.g., [64]). Clinopyroxenes in LU dunites show slightly higher REE abundances than in the both harzburgites of MTZ and mantle section ( Figure 11). In addition, the LU dunite shows adcumulate and mesocumulate textures [29] and the mineral chemistry, e.g., abrupt decrease of NiO content of olivine in harmony with Fo content of olivine (Figure 7a), also supports its cumulus origin. On the other hand, the MTZ dunite often contains olivine grains showing wavy extinction and kink bands (Figure 6b) and do not show obvious cumulus textures and depleted chemical features of minerals, e.g., high Cr# of chromian spinel (Figure 8a) and low HREE abundance of clinopyroxene than mantle residual peridotite (Figures 11b and 13). However, abrupt decrease of NiO content of olivine with decreasing Fo content of olivine implies their fractional crystalized character (Figure 7b). These features deny its simple cumulus nature. A simple residual origin is not accepted either for the MTZ dunite because of the presence of modal gap between dunite and harzburgites ( Figure 5) [65]. Nicolas and Prinzhofer [66] discussed the petrogenesis of MTZ dunite and mentioned possible replacive nature of most of the MTZ dunite in the Oman ophiolite, and the Mersin MTZ dunite is possibly of such replacive origin. The presence of chromitite in MTZ dunite of the Mersin ophiolite also supports their replacive origin based on the idea of petrogenesis of chromitite, which is a reaction product of mantle peridotite and relatively primitive basaltic magmas (e.g., [67]). On the other hand, the MTZ dunite often contains olivine grains showing wavy extinction and kink bands ( Figure 6b) and do not show obvious cumulus textures and depleted chemical features of minerals, e.g., high Cr# of chromian spinel (Figure 8a) and low HREE abundance of clinopyroxene than mantle residual peridotite (Figures 11b and 13). However, abrupt decrease of NiO content of olivine with decreasing Fo content of olivine implies their fractional crystalized character (Figure 7b). These features deny its simple cumulus nature. A simple residual origin is not accepted either for the MTZ dunite because of the presence of modal gap between dunite and harzburgites ( Figure 5) [65]. Nicolas and Prinzhofer [66] discussed the petrogenesis of MTZ dunite and mentioned possible replacive nature of most of the MTZ dunite in the Oman ophiolite, and the Mersin MTZ dunite is possibly of such replacive origin. The presence of chromitite in MTZ dunite of the Mersin ophiolite also supports their replacive origin based on the idea of petrogenesis of chromitite, which is a reaction product of mantle peridotite and relatively primitive basaltic magmas (e.g., [67]). The calculated melt in equilibrium with clinopyroxene in the LU rocks, including clinopyroxenite and wehrlite, shows lower REE abundances than the Mersin effusive rocks (tholeiitic basalt), although similar in REE pattern and concentrations to boninite, except for the slightly lower LREE abundance for the former (Figure 11b). Boninites from Izu-Ogasawara arc show variable LREE concentrations, which were interpreted to be due to the difference in amount of added slab  Figure 6. (a) Original peridotite experienced partial melting with assistance of slab-derived aqueous fluid, which was rich in LREE, LILE and Fe at relatively deeper part of mantle. (b) The original peridotite had been metasomatized by SiO 2 -rich melt and/or fluid to form secondary orthopyroxene at expense of olivine prior to partial melting event. (c) Buoyant Mg-rich metasomatized mantle with some amount of low-density partial melt started to ascend and partially melted. (c-1) In the deeper part of the upwelling diapir, the metasomatized peridotite was remelted to produce high-Mg basaltic to boninitic melt and highly depleted residual mantle harzburgite now observed in the Mersin mantle section. (c-2) In the upper part of the upwelling diapir, such Mg-rich basaltic and boninitic melt, was accumulated at the top and reacted with the roof harzburgite to produce boninite and left highly depleted dunite (MTZ rocks). The ascending boninitic melt formed cumulus LU upsection. (d) Schematic image of chemical evolution during these stages. Cr# of spinel in the Mersin ultramafic rocks has increased during partial melting and reaction stage, i.e., from panel (b) to (c), and in the fractionation stage, cumulus LU has formed from ascending boninitic melt.
The calculated melt in equilibrium with clinopyroxene in the LU rocks, including clinopyroxenite and wehrlite, shows lower REE abundances than the Mersin effusive rocks (tholeiitic basalt), although similar in REE pattern and concentrations to boninite, except for the slightly lower LREE abundance for the former (Figure 11b). Boninites from Izu-Ogasawara arc show variable LREE concentrations, which were interpreted to be due to the difference in amount of added slab component [48]. The high LREE may indicate high contributions of slab component [68]. Mineral chemistry and mineral assemblages of LU rocks, which are high-Mg# of olivine and clinopyroxene and free of plagioclase, suggests their high-pressure crystallization (≈1 GPa: [29,69]).
The melt responsible for formation of the high-Mg MTZ dunite, which contains high-Cr# spinel (Figures 7b and 8), was likely in-situ produced at the level of MTZ. No sign for the production or passage of such melt has been left in the mantle section (Figure 7). To produce a replacive MTZ dunite with a highly depleted nature (higher Cr# of chromian spinel, Fo content of olivine and lower HREE clinopyroxene), we need a highly depleted reaction system, where both or either of wall harzburgite and infiltrated melt should be depleted. The calculated melt in equilibrium with clinopyroxene in the MTZ dunite shows a similar REE pattern with boninite, although varied in LREE~MREE content (Figure 11b). This variation of REE patterns of clinopyroxene in MTZ dunite possibly indicates a variation of melt volume to the wall rock harzburgite, and the MREE-depleted spoon-shaped pattern of the calculated melt implies that the clinopyroxene had not achieved equilibration in terms of REE content with the infiltrated melt (Figure 11b). The higher Fo content of the dunite olivine in MTZ than in any harzburgite of the Mersin ophiolite implies predominance of the factor of melting of already depleted harzburgite during the interaction with infiltrated melt. Such a phenomenon possibly occurs around the top of upwelling mantle diapir of depleted peridotite, which is relatively buoyant because of the higher-Mg feature with possible presence of low-density partial melt(s) [70][71][72]. The partial melting produced high-Mg basalt to boninitic melt, possibly assisted by slab-derived hydrous components. The secondary addition of metasomatic orthopyroxene may have contributed to enhance the volume of the partial melt accumulated at the diapir top ( Figure 14). Such basaltic to boninitic melt actively reacted with the roof harzburgite overlying the diapir to produce boninite and highly depleted dunite [73]. Additionally, the ascending melt formed cumulus LU rocks upsection. The MTZ wehrlite possibly represents the melt-rich portion of the reaction zone because REE abundances are high both in clinopyroxene and the calculated equilibrium melt. The melt moved upward to formed the LU rocks upsection because the REE pattern of clinopyroxene is almost the same between the MTZ wehrlite and the LU rocks ( Figure 11).
Olivines in the mantle dunite does not show obviously low NiO content and low Fo content as observed in LU and MTZ dunites, and are plotted in the residual mantle peridotite field (Figure 7). However, the mantle dunite crosscuts the deformation structure of the mantle harzburgite ( Figure 4e) and is of replacive origin (e.g., [74]). The higher TiO 2 content of chromian spinel in the mantle dunite ( Figure 8b) and the higher REE levels of the clinopyroxenes in the mantle dunite also support its replacive origin. The calculated melt in equilibrium with clinopyroxene in mantle dunite shows almost the same pattern as that of MTZ dunite, although the abundance is higher and approaches to that of the Mersin tholeiitic basalt (Figure 11b). These characters indicate the infiltrated melt was possibly formed at much deeper part of the mantle, and the mantle dunite could be a fossil of melt conduit to feed the Mersin tholeiitic basalt upward. The variation of REE patterns of clinopyroxene in mantle dunite possibly reflects variable volume ratios of the melt to the wall rock harzburgite, and the melt volume was maybe low based on relatively high NiO content of olivine (Figure 7a) in respect of high partition coefficient of Ni in olivine relative to melt (e.g., [62]). If the melt volume was high, the NiO content of olivine decreased abruptly as observed in LU dunite (Figure 7a). The MREE-depleted spoon-shaped pattern of the calculated melt in equilibrium with those clinopyroxene implies incomplete equilibration with the infiltrated melt in terms of REE, as observed in the MTZ dunite ( Figure 11).

Implications for Tectonic Setting and History of the Mersin Ophiolite
The geochemical features of the Mersin ultramafic rocks show their similarity to the present arc-related rocks rather than MORB-related ones (Figure 8), confirming the arc affinity of the Mersin ophiolite, except for the old (Late Jurassic to Early Cretaceous) alkaline basalt [9]. The whole section of the Mersin ophiolite was formed at a suprasubduction zone condition. In addition, the boninite-related features of MTZ and lower crustal section (LU section) specify that they were formed at this level.
It is noteworthy that the MTZ is thicker (> 1 km) than in the Oman ophiolite (mostly < 500 m in thickness) [75], which recorded the switch of tectonic setting from a mid-ocean ridge to an arc [4,5].
Residual harzburgite shows highly depleted character, i.e., high Cr# of chromian spinel (0.6-0.8) (Figure 7b) and estimated high degree of partial melting (>~20%). In addition, it records Si metasomatism, which formed olivine-replacing orthopyroxene (Figure 6d) and high NiO content of remaining olivine (Figure 7a). Presence of such metasomatic orthopyroxene has been reported in peridotite xenoliths from several localities of arc setting (e.g., [1]), and is interpreted as reaction product between the peridotite and SiO 2 -rich melt or aqueous fluid derived from subducting slab. We conclude that this is one of the representative features in arc mantle. However, the lack of obvious correlation between modal amount of orthopyroxene and NiO content of olivine suggests that the formation of the secondary orthopyroxene is relatively old prior to partial melting, which obscured modal proportion of olivine/orthopyroxene and replacement texture ( Figure 14).
The partial melting was due to upwelling of Mg-rich and melt-retaining buoyant peridotite enriched with orthopyroxene and was also assisted by LREE-, LILE-, and Fe-rich slab-derived fluid. The melting of the upwelling diapir produced Mg-rich basaltic-boninitic melts ( Figure 14). The basic geochemical features of the residual mantle harzburgite of the Mersin ophiolite were obtained in this stage, and some modifications have occurred only around the discordant dunite or pyroxenite dikes. The produced melt was accumulated at the top, where the degree of partial melting is highest, and interaction with the roof harzburgite occurred to produce boninite and dunitic peridotites ( Figure 14). The MTZ is representative of the reaction zone. The boninitic melt formed by the reaction migrated to shallower part and formed cumulus LU rocks. The ultramafic rocks cumulated from the LU rocks downsection of the Mersin ophiolite are free of plagioclase [29], which indicates the formation of such boninitic magma at a pressure higher than 1 GPa.
After the formation of the Mersin ophiolite sequence, i.e., from residual mantle to ultramafic-mafic cumulates, the production of arc-type lavas (IAT tholeiite) has been a main volcanic event and such tholeiitic magmas formed discordant dunite (mantle dunite) in the mantle harzburgite (harzburgite in mantle section). Many diabase dikes with tholeiitic affinity have been observed in all levels of the ophiolite, including metamorphic sole [10], suggesting the continuous IAT tholeiite production until complete the ophiolite obduction.